Immune Response
In immune responses, the ultimate target is an antigen, (bacterium or other invader). Antigen cells, such as macrophages, ingest antigens and fragment them into antigen peptides. Parts of these peptides join to form major histocompatibility complex (MHC) molecules and display themselves on the surface of the cell. T-Lymphocytes have receptors which can recognize a non-native peptides combined with MHC molecules. T-cells are activated and secrete lymphokines, or chemical signals, that mobilize other components of the immune system. Those cells are for the most part B-lymphocytes. B-lymphocytes recognize portions of antigens in various solutions of the body, in which the antigens are not combined with MHC molecules. T-cells cannot recognize the entire antigen. The receptors on T- cells recognize protein fragments of antigens, generally peptides composed of 8 to 15 amino acids.
Consequently, there are two kinds of immune response: (1) Humoral immunity, which occurs through action of B-cells; and (2) cell mediated immunity which occurs through T-cells.
In the case of viral infection, the virus might be able, through mutation, to change its outer envelope rapidly and thus prevent neutralization by antibodies. But when the virus contains proteins within its core that are essential for its life process, that mutation is not permitted. When the virus replicates inside cells, short peptide chains break off from the viral proteins and travel to the cell surface. They serve as ripe targets for the T-cells, which can then attack the infected cell and inhibit the spread of the virus, or as happens in autoimmune diseases, to attack the cells of the body.
T-cells themselves comprise two subpopulations, CD4 (helper) and CD8 (killer) cells. Each type of T-cell uses its own form of MHC to make peptides recognizable. After CD4 cells receive the proper chemical signal, they produce large amounts of lymphokines to accelerate the division of other T-cells. Activated CD8 cells produce much smaller amounts of lymphokines but develop the capacity to punch holes into target cells and to secrete chemicals that kill infected cells or cells which have been in some other way changed.